Alternative Names

How the Test is Performed

The most common glucose tolerance test is the oral glucose tolerance test (OGTT).

Before the test begins, a sample of blood will be taken.

You will then be asked to drink a liquid containing a certain amount of glucose (usually 75 grams). Your blood will be taken again every 30 to 60 minutes after you drink the solution.

The test may take up to 3 hours.

A similar test is the IV glucose tolerance test (IGTT). It is rarely used, and is never used to diagnose diabetes. With IGTT, glucose is injected into your vein for 3 minutes. Blood insulin levels are measured before the injection, and again at 1 and 3 minutes after the injection. The timing may vary.

How to Prepare for the Test

Make sure you eat normally for several days before the test.

Do not eat or drink anything for at least 8 hours before the test. You cannot eat during the test.

Ask your health care provider if any of the medicines you take can affect the test results.

How the Test will Feel

Drinking the glucose solution is similar to drinking very sweet soda.

Serious side effects from this test are very uncommon. With the blood test, some people feel nauseated, sweaty, light-headed, or may even feel short of breath or faint after drinking the glucose. Tell your doctor if you have a history of these symptoms related to blood tests or medical procedures.

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing.

Why the Test is Performed

Glucose is the sugar the body uses for energy. Patients with untreated diabetes
have high blood glucose levels.

Most often, the first tests used to diagnose diabetes in persons who are not pregnant are:

Fasting blood glucose level -- diabetes is diagnosed if it is higher than 126 mg/dL on two different tests

Hemoglobin A1c test -- diabetes is diagnosed if the test result is 6.5% or higher

Glucose tolerance tests are also used to diagnose diabetes. The oral glucose tolerance test is used to screen for, or diagnose diabetes in people with a fasting blood glucose level that is high, but is not high enough (above 125 mg/dL) to meet the diagnosis for diabetes.

Normal Results

Normal blood values for a 75-gram oral glucose tolerance test used to check for type 2 diabetes in those who are not pregnant:

Fasting: 60 to 100 mg/dL

1 hour: less than 200 mg/dL

2 hours: less than 140 mg/dL

The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

A glucose level than is higher than normal may mean you have pre-diabetes, diabetes, or gestational diabetes.

Between 140 and 200 mg/dL is called impaired glucose tolerance. Your doctor may call this "pre-diabetes." It means you are at increased risk of developing diabetes over time.

For help in finding a doctor or health service that suits your needs, call the UPMC Referral Service at 412-647-UPMC (8762) or 1-800-533-UPMC (8762). Select option 1.

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